1. Field of the Invention
This invention relates to waveguide lasers and, more particularly, to double heterostructure diode lasers constrained to lase in a single filament mode.
2. Description of the Prior Art
For many diode laser applications a single spot laser beam is very desirable. Preferably, the width and height dimensions of the spot should be as nearly equal as possible, the divergence as low as possible, the power as high as possible, and the spatial location as stable as possible. What is desired in theory is single filament lasing at all injection levels up to the catastrophic failure level. However, while a diode laser normally begins lasing at threshold in a single filament or spot, the tendency at higher pump current levels is for the single filament to break up into a multifilament configuration. No distinction will be made herein between multimode lasing configurations and a single higher order mode lasing configuration, since both have effectively more than one lasing filament and therefore both may be considered multifilament lasing configurations. "Single filament" herein implies a single lasing mode with a generally elliptical transverse intensity distribution which resembles the fundamental transverse mode of a two-dimensional dielectric waveguide. Prior art structures for single filament lasing typically have been unsuccessful at high injection levels, with the more successfully operating structures being the most difficult to fabricate in general.
Conventional prior art double heterostructure (DH) diode lasers normally have carrier and optical confinement only in the direction perpendicular to the p-n junction (hereinafter called the transverse direction). The lasing mode is thereby controlled in the transverse direction (hereinafter called the transverse lasing mode), but not in the direction parallel to the junction (hereinafter called the lateral direction). As a result, more than one lasing filament ordinarily occurs as soon as the injection level is raised substantially above threshold.
Conventional stripe geometry DH lasers provide some control of the lasing mode in the lateral direction (hereinafter called the lateral lasing mode) by controlling the lateral current distribution and therefore the lateral gain distribution with the stripe electrode. However, a minimum width for the stripe electrode is established by heat dissipation requirements. The current spreads over a still wider area before it reaches the active layer resulting in a lateral gain distribution which is still too wide to produce single filament operation except at current levels close to threshold.
In U.S. Pat. No. 3,883,821, the active layer in a stripe geometry DH laser is given a rib configuration, which results in an optical confinement effect in the lateral direction which, it is predicted, will produce single filament operation. Both optical and carrier confinement in the lateral direction occur in the buried heterostructure laser described by T. Tsukada in the Journal of Applied Physics, Vol. 45, No. 11, (November 1974) at pages 4899-4906 and entitled "GaAs-Ga.sub.1-x Al.sub.x As Buried-Heterostructure Injection Laser." Single filament operation again occurs because the transverse and lateral modes are confined within a region of suitably small geometry. However, fabrication of both of these structures requires at least two separate epitaxy growth steps separated by at least one etching step. As a practical matter, this produces a growth interface having detrimental effects.
Devices which may be fabricated using a substantially continuous epitaxial deposition have a clear fabrication advantage and avoid detrimental epitaxy interface problems. One such device is disclosed in an article entitled "Single Mode Operation of GaAs-GaAlAs TJS-Laser Diodes" published by H. Namizaki in the Transactions of the IECE of Japan, Vol. E59, No. 5 (May 1976) at pages 8-15. However, the disclosed device suffers from the disadvantage that there is a zinc diffusion, which in practice is difficult to control to the degree required. Zinc diffused GaAs is also known to have a tendency toward developing crystal defects.
In U.S. Pat. No. 3,978,428 a continuous epitaxial process is used to produce the active and confining layers in an etched groove. Optical confinement in the lateral direction occurs because the active region becomes thinner in the lateral direction. The stripe contact and the diffused layer at the shoulders of the groove also act to control the lateral current and thereby gain distribution. In practice, however, the diffused layer at the shoulders tends to short circuit the active region because the lower confining layer at the shoulders becomes too thin. If the lower confining layer is made thicker at the shoulders, then the groove tends to be filled in so much by this layer that device performance suffers for other reasons.
In the article entitled "Improved Light-output Linearity in Stripe-Geometry Double-Heterostructure (Al,Ga)As Lasers," Applied Physics Letters, Vol. 29, No. 6 (September 1976) at pages 372-374, Dixon, et al, describe a stripe having a reduced width. The region outside the desired active area is also proton bombarded ("proton-delineated" stripe region) so that the current which laterally spreads into the bombarded region does not produce any effective minority carriers, thereby controlling the gain distribution in the lateral direction. As reported in this article, single filament lasing results from this configuration over an improved current range, but high current operation still results in a multifilament configuration.
In an article entitled, "Channeled-Substrate Planar Structure (AlGa)As Injection Lasers," Applied Physics Letters, Vol. 30, No. 12 (June 1977), Aiki, et al, describe a structure in which the lateral active regions are effectively made more lossy in order to create an effective reduction in index of refraction in the lateral directions. The resulting effective optical confinement controls the lateral mode, but only at moderate injection levels.